Project Summary Triple-negative breast cancers (TNBC) occur in 10-15% of patients, yet this disease subtype accounts for almost half of all breast cancer deaths. TNBCs lack clinical expression of estrogen receptor (ER)-alpha, progesterone receptor and HER-2 and cannot be treated with current endocrine or HER2-targeted therapies. TNBCs are heterogeneous and occur often in African American and younger women. Although initially responsive to some chemotherapies, TNBCs tend to relapse and metastasize early, leading to poor clinical outcome. Our highest priority is to define new therapeutic targets that can be translated to the clinic. We predict that expression of a second ER gene product, termed ER-beta, may be such a target in all or subsets of TNBCs. In preliminary work with TNBC biopsy specimens from African American and Caucasian women, we find ER-beta1 expression correlates with significantly worse 5-year overall survival in patients treated with chemotherapy and with shorter disease-free intervals. Further, a panel of TNBC cell lines all exhibit significant ER-beta expression, with relatively higher levels detected in cells from African Americans. Other reports also find ER-beta associates with more aggressive phenotypes in breast cancer, particularly in African Americans. Thus, we hypothesize that ER-beta occurs in most TNBCs and modulates tumor progression. In addition, preliminary data show that insulin-like growth factor-2 (IGF-2), known to associate with tumor promotion, is also expressed in TNBC specimens and stimulates high expression of ER-beta mRNA to promote TNBC progression. Growth-stimulating effects of this pathway may be due in part to downstream actions to promote VEGF, amphiregulin and Wnt-10b activity. Aims of this project are to: 1) use tissue microarrays (TMA) to assess expression of ER-beta forms and IGF-2 as well as clinical consequences of expression in individuals of diverse race/ethnicity with TNBC; 2) assess effects of ER-beta on cell proliferation, apoptosis and signaling interactions with growth factors (IGF-2) and PD-L1 in TNBC cells in vitro; 3) determine activity of available ER- beta agonists and antagonists on progression of human TNBC implants in immunodeficient mice in vivo. This work may help address the unequal burdens of cancer among racial/ethnic minorities and underserved groups represented in the TNBC-TMA and assess the interplay of these factors with cancer biology?namely expression and activity of important hormone/growth factor signaling pathways in patients afflicted with TNBC.